Current neuropsychological models propose that the cognitive changes associated with healthy adult ageing are due to deterioration of the frontal lobes of the brain. Despite evidence that the frontal lobes are involved in age-associated cognitive decline, the behavioural and cognitive deficits demonstrated by older adults differ from the typical clinical picture presented by patients with frontal lobe damage. Furthermore, there are frontal lobe tests reported in the literature that are insensitive to the effects of healthy adult ageing despite being sensitive to the effects of frontal lobe dysfunction. These arguments speak against the current "frontal lobe hypothesis of ageing". Studies have demonstrated that the frontal lobes can be subdivided into at least two distinct areas: the dorsolateral and ventromedial prefrontal regions. Current neuropsychological models of ageing have failed to consider that age may differentially affect these regions and assume that there is uniform frontal decline. Autopsy and neuroimaging studies, however, suggest that the dorsolateral region deteriorates earlier and more rapidly than the ventromedial region. Therefore, the aim of this thesis was to outline and test a "dorsolateral" prefrontal theory of cognitive ageing where the dorsolateral functions deteriorate with age earlier and more rapidly than the ventromedial functions. In a series of experiments, age-associated declines in performance were found on all tasks sensitive to dorsolateral prefrontal dysfunction, but not on the majority of tasks sensitive to ventromedial prefrontal dysfunction. An attempt was also made to provide evidence for the specific localisation of the "dorsolateral" and "ventromedial" measures by assessing groups of patients with lesions to different areas of the frontal lobes. Whilst most of the tasks were sensitive to frontal lobe dysfunction, only two of the "dorsolateral" measures were found to be selectively sensitive to the dorsolateral prefrontal region. In conclusion, the profile of spared and impaired abilities in the older groups speaks against the traditional "frontal lobe" interpretation of cognitive ageing and is more supportive of a specific dorsolateral prefrontal theory of cognitive changes with age.